1. Field of the Invention
The present invention generally relates to a spread spectrum global positioning system (GPS) receiver, and more particularly to a method and apparatus capable of reducing the size of a memory that stores n bits correlation integral values during a synchronization acquisition procedure in the GPS receiver.
2. Description of the Related Art
A wide-area GPS is provided with 30 or more satellites in particular orbits. Twelve satellites can belong to one signal coverage and can communicate simultaneously with a GPS receiver at a specific position.
The GPS receiver determines the GPS receiver's position by computing the relative times of arrival of signals transmitted simultaneously from a number of GPS satellites to determine the receiver's distances from the satellites. The GPS satellites transmit, as part of the satellites' message, both satellite positioning data including pseudo-noise (PN) codes and data on clock timing. In order to calculate the position, the GPS receiver has to receive signals of at least four visible satellites within the same signal coverage.
Using the received PN codes, the GPS receiver determines pseudo-ranges to the various GPS satellites, and computes the position of the GPS receiver using the pseudo-ranges and satellite timing and data on clock timing. The satellite data on clock timing and signature data are extracted from the GPS satellite signals once a satellite is acquired and tracked.
Each of the GPS satellites transmits an L1 signal having a carrier frequency of 1575.42 MHz. The frequency of the L1 carrier signal is represented by 154f0, where f0=10.23 MHz.
In the satellites, signals are modulated with high rate repetitive signals called pseudo noise sequences (PN codes) according to a code division multiple access (CDMA) scheme and are formed into code modulation wideband signals.
One of the pseudo noise sequences used in the respective satellites to modulate the L1 signal is a coarse-acquisition (C/A) code. The C/A code sequences belong to a family known as Gold codes. Each GPS satellite broadcasts a signal with a unique C/A code. The codes are formed by execution of a modulo-2 addition of two 1023-bit binary sequences. The C/A codes are binary codes and have a binary phase-reversal rate, or “chipping” rate, of 1.023 MHz and a repetition period of 1023 chips for a code period of 1 millisecond.
The carrier of the L1 signal is further modulated with navigation information having a bit rate of 50 bits/s. The navigation information includes various information of the GPS satellite such as health, orbit, clock data parameter related with the GPS satellite and so forth.
The GPS receiver performs a synchronization operation for detecting satellite signals to identify the GPS satellites. Further, the GPS receiver searches signals transmitted from the respective GPS satellites to perform the synchronization operation with the signals transmitted from the GPS satellites so that data transmitted as the signals can be received and demodulated.
The GPS receiver needs to perform the synchronization, for example, in the situation in which the receiver is turned on and the receiver cannot receive any satellite signal during a predetermined time period. Such a situation may frequently occur because a mobile device having the receiver may be moving and thus it is difficult for an antenna of the mobile device to be always located at the most suitable place for receiving the satellite signals, and thus the strength of the signals transmitted to the receiver is weak. In urban areas, buildings influence the satellite signals received from the GPS satellite, and the satellite signals undergo multi-path propagation, in which the satellite signals from the GPS satellites are transmitted to the GPS receiver through various routes such as a direct transmission route from the GPS satellite and different routes that are caused by the satellite signals being reflected by tall buildings. Due to the multi-path propagation, the received signals have time differences and phase differences that can cause errors in determination of position of the receiver.
A distance between the GPS receiver and the GPS satellites is called a pseudo-range due to a clock error between the GPS satellite and the GPS receiver and a signal delay through the atmosphere layers.
The pseudo-ranges correspond to time delay values measured between the received satellite signals from each of the satellites and a local clock signal of the GPS receiver.
Determination of position and time is repeated until sufficient accuracy is achieved.
Pseudo-range calculation is performed by measuring an average of the transmission time of different satellite signals. After the GPS receiver synchronizes with the received signals, information transmitted through the signals is demodulated.
Most GPS receivers use correlation methods to calculate the pseudo-range. Pseudo noise sequences of the GPS satellites are locally stored or produced in the GPS receiver.
Down-conversion for the received satellite signal is performed, and the GPS receiver correlates the down-converted signal with the locally stored or produced pseudo noise (PN) sequences. The correlated result is integrated. The correlation integral value (or sampling value) indicates the presence of the satellite signal in the received satellite signal. The correlation operation executed in the GPS receiver is repeated so that a phase of the pseudo noise sequence stored in the receiver is shifted. The phases of the PN sequence are tracked until an accurate phase is obtained. The accurate phase is obtained when a correlation result is the highest.
The synchronization and phase (or frequency) adjusting processing is repeated for respective satellite signals received in the receiver. Therefore, such processing time is very long in a situation in which the strength of the received signals is weak.
In conventional GPS receivers, several correlators are used to accelerate the processing speed, so that more correlation peaks can be searched at the same time. However, the number of correlators cannot increase infinitely, and thus there is a limitation to accelerating the synchronization and the phase (or frequency) adjusting process by simply increasing the number of the correlators.
Generally, the receiver has a plurality of channels for the sake of rapid synchronization, and the respective channels include multiple correlators. The number of correlation integral values stored in a memory of the GPS receiver increases in proportion to the number of channels and correlators, and thus a large amount of storage space is needed. In addition, in order to perform an algorithm for determining synchronization acquisition in a processor based on the data stored in the memory, data access amount between the processor and the memory is increased according to the number of correlation integral values stored in the memory increases.
Thus, in order to store a large amount of data in the memory at a high speed and in order to read out the data from the memory at a high speed, an increased memory access time is required. The memory access time is an important factor affecting acquisition speed and thus performance of the GPS receiver.
There is a need for a large capacity memory for storing a lot of data for the acquisition in the GPS receiver, and thus a size of the memory increases, and thus it is difficult to reduce the size of the GPS receiver.